Method and apparatus for the continuous production of prestressed concrete members



Sept- 25, 1962 B. c. GERWICK, JR 3,

METHOD AND APPARATUS FOR THE CONTINUOUS PRODUCTION OF PRESTRESSEDCONCRETE MEMBERS- Filed Nov. 6, 1959 j w I.

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BY 2Xmbu ATTORNEY United States Patent 3,055,073 METHOD AND APPARATUSFOR THE CONTINU- OUS PRODUCTION OF PRESTRESSED CON- CRETE MEMBERS Ben C.Gerwick, Jr., Oakland, Calif, assignor to Ben C. Gerwick, Inc., SanFrancisco, Calif., a corporation of California Filed Nov. 6, 1959, Ser.No. 851,464 6 Claims. (Cl. 25-2) This invention relates to theproduction of reinforced, prestressed concrete by a continuous process.

Prestressed concrete members are widely used throughout the world, andare available in a great variety of shapes for many purposes (e.g.,bridge girders, pilings, building columns and girders, floor and roofslabs, railroad ties, power poles, etc.). As generally employed in theUnited States, the pretensioned process involves the use of a long bedin which wires or strands are stretched. Abutments four hundred feet ormore apart may be employed. Forms are placed about the Wires, andconcrete poured inside the forms to embed the wires in the concrete.Upon curing or hardening of the concrete, the forms are removed and thewires released, the wires gripping the concrete by bond to shorten andprestress the concrete.

Where concrete members of predetermined lengths are desired, the generalpractice is to employ a plurality of forms or molds positioned end toend about the reinforcement. Upon cutting the wires between the units,the separate units may be lifted free and transported to the site forerection.

Some attempts have been made heretofore to produce prestressed concretemembers by a continuous process. In every case however, it has beennecessary to employ a plurality of mold forms or boxes which are coupledtogether to form a train. Such system has been used extensively, forexample, in the U.S.S.R. In this country, the mold forms are consideredexpensive and hard to use (requiring continuous attention if properalignment is to be achieved) and consequently have not found greatfavor.

In general, it is an object of the present invention to provide a methodand means by which a truly continuous production of pretensionedconcrete products can be obtained.

Another object of the invention is to provide a procedure of thischaracter in which all materials are supplied at approximately the samepoint, work is done on the concrete as it moves, and completed concreteproducts are continuously removed from the process for shipment, storageor use.

Another object of the invention is to provide a pro-- cedure of thischaracter which eliminates the requirement for separate mold forms.

Additional objects and advantages of the present invention will beapparent from the following description of a preferred embodiment, andfrom the drawings in which:

FIGURE 1 is a view in plan of a system for continuously producingprestressed concrete members in accordance with the invention;

FIGURE 2 is a view in side elevation thereof;

FIGURE 3 is an enlarged view in section along the line 33 of FIGURE 1;and

FIGURE 4 is a like view along the line 4-4 of FIG- URE 1.

Generally stated, my invention is directed to a procedure for thecontinuous manufacture of prestressed concrete wherein reinforcementstrands are passed through the length of a continuously moving moldform, simultaneously with filling of the form with a fresh mix ofconcrete. The concrete and embedded reinforcement are 3,055,073 PatentedSept. 25, 1 962 supported for movement within the mold form until suchtime as the concrete is hardened sufficiently to be self-supporting. Thehardened concrete is then grasped and pulled out of the mold in thedirection of movement to thereby maintain the reinforcing strands undera predetermined desired tension. After it has set, the continuous lengthof reinforced concrete can be cut to desired lengths, with release ofthe tension on the reinforcement shortening and prestressing thefinished concrete products.

Referring to the drawings, 10 represents a continuously moving mold formwhich in a typical instance may consist of a bottom endless belt 12 anda pair of endless side belts 14. In the preferred embodimentillustrated, the inner reach of each endless belt is supported within aframe 16 of desired cross sectional configuration. The belts 12, 14, andthe frame 16 together form an elongated mold form, the inner walls ofwhich are continuously moving (i.e., to the left in FIGURES 1 and 2).

Adjacent the inlet end of the mold form, a plurality of reels 18 ofreinforcement strands or wire are supported on a common unwinding axle20. Preferably, brake means 22 are associated with the axle so that thestrands 24 being pulled off the reels may be maintained under apredetermined uniform tension. The various strands are fed through apattern or template 26, through the mold form 10, and thence to the faror discharge end of the production line.

In accordance with the invention, a freshmix of con-' crete iscontinuously supplied to the moving belts forming the mold 10, fillingthe same and embedding the strands of reinforcement in moving concrete.As illustrated, the concrete is supplied from a mixing station 30 whichmay include a batching substation 27 adapted to receive aggregate andcement from the stockpiles 29 and 31. From the adjacent mixingsubstation 32, mixed concrete can be moved on a conveyer 33 to a hopper34 for discharge into the mold form 10. The concrete is preferably adense, dry mix which is sufliciently self-supporting to fill the moldform approximately to the level of the endless side belts 14. Aninternal vibrator 36 may be positioned immediately beyond the dischargepoint to compact the concrete and to eliminate moisture and air pockets.One or more vibratory screeds 38 may be employed to further compact andfinish the concrete.

In operation, the endless belts 12 and 14 forming the mold form travelat a very slow rate to permit hardening or partial setting of theconcrete about the reinforcement strands. By Way of illustration, therate of travel of the belts may be such that concrete delivered from thehopper 34 remains within the form for a period of two or more hoursprior to discharge from the far end of the mold. Preferably theindividual belts are fabricated of rubber or an elastomer, such asneoprene, to which the concrete material is non-adherent. From the endof the moving mold, the belts return through a cleaning bath (not shown)located underneath the mold.

The partially hardened concrete delivered from the mold 10 isimmediately received on a conveyer 40 which may be an endless steelbelt, as illustrated in FIGURE 4, or a plurality of transverse rollerconveyers, or other suitable means. The conveyer 40 functions tomovingly support the concrete as it passes into a curing tunnel 44,where it is subjected to the curing eifect of moist steam. This steam ispreferably within the temperature range from about F. to 170 F. and issupplied from a boiler or other suitable source 46 through the manifold48. Preferably the rate and conditions in the tunnel 44 are such thatthe concrete is initially exposed to moist air of little or no sensibleheat fora period of about two hours. Thereafter the temperature of theconcrete is progressively raised to the order of F. in about one hour,with the total curing time in the tunnel ranging from six to twentyhours. During the last hour or so in the tunnel, the concrete may beallowed to cool to atmospheric temperature. The concrete emerging fromthe curing tunnel 44 is of suflicient strength and hardness to firmlygrip the reinforcement strands in bond.

I have found that the concrete leaving the curing tunnel can bepositively grasped and advanced against the pull of the braking means 22to place the reinforcement under desired prestressing tension. Asillustrated at the station 50 (FIGURE 4), preferred means for thispurpose include a pair of resilient roller elements 51 whichfrictionally engage the sides of the concrete to exert a constant andcontinuous longitudinal pull upon both the wires 24 and the concrete.These rollers can be two extremely large pneumatic tires that squeezethe product with suflicient force to provide the necessary frictionalengagement. Any suitable means such as the motor 52 and gear train 54,56 may be employed to rotate the rollers, provided it exerts alongitudinal pull of sufficient magnitude. By way of illustration, apull of 80 to 200 psi. (corresponding in a typical instance to a totalpull of 40 to 100 tons or more) has been found to be satisfactory. Theadvantage of the resilient roller elements 51 is that they not onlyexert sufficient longitudinal pull but also permit easy passage of themoving concrete.

It will be understood that other devices to grasp and move the concretemay be employed, for example, continuous or practically continuousjacking devices (inflatable or of other types) which similarly employfriction to advance the concrete. However, the resilient roller meanshave been found to possess a number of advantages, and are preferred.

"As soon as the continuous concrete body 8 has been engaged by thefriction device at 50, it can be subjected to operations designed tosever desired lengths of reinforced concrete from the moving mass.Illustrative of such apparatus is the traveling saw 60, which may be ofconventional design. \As will be understood, apparatus of this typetravels a short distance with the product during the cutting operation,to insure a square cut.

An alternative to the method just described (e.g. manufacturing acontinuous member and then cutting it to length) is to feed dividersinto the mold form 10 at proper intervals in the manufacturing process.This could be accomplished .by mounting a stack of the dividers suitablyapertured to receive the reinforcement adjacent the template 26, andprior to commencement of operations. The dividers could then be releasedand fed into the mold form as desired.

The overall operation of the apparatus just described can now besummarized as follows: To initiate operations, reinforcement strands 24are unwound by a mechanical means from the reels 18 and against theresistance of the braking means 22. In general, it is desirable to carrythe strands at least as far as thecuring tunnel 44. Once production isunder way, the force for pulling the reinforcement oli the reels isderived from movement of the hardened concrete itself, as effected bythe gripping means 51 at the station 40. Cement and aggregate from thestockpiles at station 30 are now batched and mixed and delivered throughthe conveyor 32 to the hopper 34. From the hopper, the freshly mixedconcrete flows into the continuously moving mold form 10 wherein theconcrete is confined by the endless belts 12 and 14 during thepreliminary compacting, screeding and finishing operations performed bythe devices 36 and 38. The concrete and embedded reinforcement strandsare moved slowly within the mold so that a certain amount of setting orhardening of the concrete occurs before discharge into the curing tunnel44. In the tunnel 44, the concrete is gradually heated, preferably bymoist steam, to a temperature of the order of 160 F. and is maintainedat this temperature for a period of time sufiicient to harden and curethe concrete. The concrete emerging from the curing tunnel isimmediately grasped by the friction devices 51 cage).

which exerts a constant and continuous longitudinal pull. Thereafter,prestressed sections of reinforced concrete 62 are severed at thestation 60, in desired lengths, and can be loaded for shipment orstorage as meets construction requirements.

A number of additional variations are possible in the procedure justdescribed without departing from the scope of the invention. Forexample, if spiral reinforcement is desired, it may be suppliedimmediately prior to entry of the reinforcement into the concrete, as atthe station 64. Apparatus for this purpose may be of conventional design(e.g., a steel cage rotating transversely about the strands 24 to unwindreinforcement from the inside of the Special inserts (for example, asneeded in the manufacture of railroad ties) can also be mechanicallyplaced either before or after pouring. A further modification would beto provide blisters or humps on the inside of the endless belts 12 or 14so as to provide variations in the cross section of the concrete atregular intervals. External vibration may also be applied, for example,by operation of mechanical vibrators on an external surface of the frame16. These and other variations are clearly within the skill of oneversed in this art.

I claim:

1. A method for the continuous manufacture of prestressed concretecomprising the simultaneous and progressive steps of continuouslydischarging freshly mixed concrete into an elongated pouring zone,continuously feeding reinforcement strands lengthwise into said pouringzone to embed the same in said concrete, continuously conveying thefreshly discharged concrete and embedded reinforcement strands away fromsaid pouring zone with support of said concrete along its bottom andsides until the concrete has set sufficiently to hold said reinforcementstrands embedded therein, curing said partially set concrete to form anelongated mass of hardened concrete, continuously frictionally engagingsurface portions of said hardened concrete to pull and convey the sameas an elongated hardened mass away from said pouring zone, saidfrictional engagement and resulting conveyance serving to place saidreinforcement strands within the freshly mixed concrete underprestressing tension, and severing desired lengths of the elongatedhardened reinforced concrete to thereby release the reinforcementstrands to shorten and prestress the severed lengths of hardenedconcrete.

2. In apparatus for the continuous manufacture of elongated p'restressedconcrete units, means forming a continuously moving mold form, saidmeans comprising an elongated frame of desired cross sectionalconfiguration, said frame including a bottom and sides, endless beltmeans reeved through said frame to provide continuously moving bottomand side surfaces of said mold form, means to continuously feedreinforcement strands lengthwise into an inlet end of said continuouslymoving mold form, means adjacent said inlet end of the mold form tocontinuously charge a fresh mix of concrete to said inlet end and aboutsaid reinforcement strands, means advancing said endless belts at a veryslow rate to permit time within the mold for the concrete to partiallyharden, additional elongated conveyer means adjacent to a discharge endof said mold form to receive and support the partially hardenedconcrete, means advancing said additional conveyor means at a very slowrate to permit time for further hardening and curing of said concrete,resilient means adjacent to said additional conveyor and spaced asubstantial distance from an inlet end thereof to frictionally graspsurface portions of the cured concrete carried by said additionalconveyer means, said resilient means being aligned with said additionalconveyer means and cooperating with the same to pull the concrete fromsaid discharge end of said mold form, and means to sever desired lengthsof concrete and reinforcement strands whereby the latter function toshorten and prestress the severed lengths.

3. Apparatus as in claim 2 wherein said reinforcement strands aresupplied through braking means positioned adjacent said inlet end of themold form and adapted to maintain said strands under predeterminedtension.

4. Apparatus as in claim 2 wherein tunnel means are provided about saidadditional conveyor means and between said moving mold form andresilient friction means, said tunnel means including means to raise thetemperature of the moving concrete to a temperature of the order of 160F. to effect curing of the same.

5. Apparatus as in claim 2 wherein said resilient means comprisesresilient roller elements rotatable about vertical axes and adapted tofrictionally engage the sides of the hardened, cured moving concrete.

6. Apparatus as in claim 2 including winding means between the inlet endof said mold form and said means supplying reinforcement strands, saidwinding means wrapping spiral reinforcement about said strands.

References Cited in the file of this patent UNITED STATES PATENTS1,367,227 Baumgartl -2 Feb. 1, 1921 1,956,967 Upson May 1, 19342,607,099 Schroder et al Aug. 19, 1952 2,757,415 Mathues et al. Aug. 7,1956 2,912,738 Bergling et a1 Nov. 17, 1959 FOREIGN PATENTS 723.009Great Britain Feb. 2, 1955

